1. Field of the Invention
The field of the invention is non-volatile memory and integrated circuits with non-volatile memory generally, and more specifically, to operating methods and circuitry for the non-volatile memory.
2. Description of Related Art
Read disturb presents an increasingly serious problem to the operation of non-volatile memory cells in general, and charge-trapping memory cells in particular. Read disturb occurs when read operations are carried out on the memory cell. During a read operation, a drain bias such as 1.6 V can cause current flow from the source to the drain of the memory cell. The drain bias can be sufficiently high to add electrons to the trapping part of the charge-trapping structure by the drain. After repeated read operations, these added electrons raise the threshold voltage of the trapping part of the charge-trapping structure by the drain.
FIGS. 1A and 1B show the read disturb problem in memory cells. Both FIGS. 1A and 1B show a charge-trapping memory cell with a substrate 140, first current-carrying terminal 150, second current-carrying terminal 160, bottom oxide 130, charge-trapping structure 120, top oxide 110, and gate 105. The gate 105 has a 4V bias. Depending on which of the first current-carrying terminal 150 and the second current-carrying terminal 160 has a higher voltage, either can act as the drain.
In FIG. 1A, the first current-carrying terminal 150 has a bias of 0V and the second current-carrying terminal 160 has a bias of 1.6V. In this read bias configuration, electrons 173 move along path 171 from the first current-carrying terminal 150 through the substrate 140 toward the second current-carrying terminal 160, transit the bottom oxide 130, and are trapped in charge-trapping structure 120.
In FIG. 1B, the first current-carrying terminal 150 has a bias of 1.6V and the second current-carrying terminal 160 has a bias of 0V. In this read bias configuration, electrons 177 move along path 175 from the second current-carrying terminal 160 through the substrate 140 toward the first current-carrying terminal 150, transit the bottom oxide 130, and are trapped in charge-trapping structure 120.
A serious consequence of read disturb is that, after multiple read operations, the trapping part of the charge-trapping structure by the drain which originally stored the low threshold state instead stores a high threshold state, due to read disturb. In a less extreme consequence of read disturb, the trapping part of the charge-trapping structure by the drain which originally stored the low threshold state instead stores an indeterminate state—the threshold voltage of the trapping part of the charge-trapping structure by the drain is raised to a voltage above the upper limit of the low threshold state (e.g., above erase verify) but below the lower limit of the high threshold state (e.g., below program verify). Accordingly, what is needed is a solution to the read disturb problem of charge-trapping memory cells.